This is only used when a wavelength-dependent model is being used to predict sky intensities using whole-order polynomials. In this case, any pixel whose intensity differs from the fit by more than sigma TUNE_SKYLTHR will be treated as a sky-line candidate, and its actual intensity entered into the model in place of the fit prediction. The same procedure is also applied to the pixels' TUNE_SKYLINW nearest neighbours to ensure that the entire sky line is entered into the model even when its wings are within the sigma constraint described above. This parameter only has effect when TUNE_SKYXPLY>0.
The default value is 5 pixels.